A Prograde Dual-Segmented Geotherm for (Retro-) Eclogite From Western Dabie and Implications for Maximum Decoupling Depths During Continental Subduction

Abstract

Robust quantification of the prograde P–T trajectories of eclogite exhumed from subduction zones is fundamental for deciphering the thermal structure evolution and understanding the geodynamic processes during continental subduction. In this study, we investigate four metabasites located in Hong'an within the western Dabie HP/UHP metamorphic belt. Based on detailed petrographic observations and mineral chemistry analyses, combined with phase equilibrium modelling, average-T calculation and conventional thermobarometry, we quantify the prograde to peak P–T paths for each of the four metabasites and the retrograde P–T conditions for two samples. The results show that three of the four metabasites have similar prograde P–T paths evolving from 15.5–18.5 kbar, 440–485 °C (M0 stage) to 18.5–20.5 kbar, 500–525 °C (M1 stage). On the other hand, although the fourth sample shares a similar P–T evolution for a segment of the late prograde stage from 18.0–19.0 kbar, ~500 °C to 20.0–22.0 kbar, ~550 °C, it attains P_max at a notably higher pressure of ~26.0 kbar at 550–560 °C (M2 stage). During exhumation, we identify an early retrograde stage occurring at 9.0–12.5 kbar, 545–580 °C (M3 stage), followed by a later retrograde stage at 3.5–8.0 kbar, 540–580 °C (M4 stage). In combination with previous studies, we propose a common dual-segmented P–T path for the late prograde evolution of the HP/UHP rocks in western Dabie. The initial segment exhibits a gentle slope with apparent geotherms of 7–8 °C/km, whereas the subsequent segment displays a steeper slope with apparent geotherms of 5–6 °C/km. We interpret the turning point at 20.0–23.0 kbar (corresponding to depths of 70–80 km) as marking the maximum decoupling depths (MDD) between the subducting slab and the overlying mantle wedge. Notably, this prograde dual-segmented geotherm for eclogite in western Dabie and the corresponding MDD are similar to computational geodynamic models.